This invention relates to a blank for use in manufacturing fiber-reinforced coatings or metal components, comprising fibers made of a high-strength material and metals of a lower strength than the fibers, as well as a process for manufacturing the blank, the coatings or the components.
U.S. Pat. No. 3,936,550, discloses a process for manufacturing fiber-reinforced coatings or metal components comprising fibers of a high-strength material and titanium or titanium alloys as the metal. For this purpose, the fibers are isostatically pressed between two titanium foil layers at high-temperature. However, because the fibers are not fixed in their position between the foil layers, they may become superimposed on one another or contact one another, during compression and therefore weaken the composite material at these points. Furthermore, such fibers and foil layers are relatively stiff, and cannot be applied or formed onto components or cores before high-temperature isostatic pressing. In addition, the fibers consist of boroxides which lose their stability even at a low thermal stress, and undergo a plastic deformation.
Another disadvantage of the known blank made of titanium foil layers and boroxide fibers is that, when the blank is compressed to form fiber-reinforced metal plates, the packing density of the fibers is limited. For components having several fiber layers above one another, when the blank disclosed in U.S. Pat. No. 3,936,550 is used, a metal foil layer is disposed between each fiber layer, which limits the packing density of the fibers. When the packing density of the fibers within one fiber layer is high, there is the risk that insufficient metal material will penetrate between the fibers during the compression to form a fiber-reinforced metal; thus pores, holes, or cavities which are free of matrix metal can remain in the composite material, weakening it and posing an increased breakage risk for the fibers.
It is an object of the present invention therefore to provide a blank of the above-mentioned type which can be formed onto components or cores without shifting of the fiber position, and without forming pores or cavities. The blank according to the invention therefore yields fiber-reinforced metal components with a high packing density of the fibers, and permits thermal pressing processes without any damage to the fibers of the blank.
According to the invention, this object is achieved in that the fibers are aligned parallel to the long axis of the blank, are embedded in a spaced manner in knitted metal wire meshes, and are stable with respect to high temperatures.
The blank according to the invention has the advantage that it is extremely flexible mechanically and in fact has superior flexibility compared to woven or felt structures. It can therefore be applied in a wider range of environments, and permits fiber-reinforced metal structures which up to now could not be achieved, by pressing or compressing of blanks according to the invention. The packing density of the fibers is limited only by the wire gauge of the knitted metal wire meshes. A blank according to the invention which is compressed to form a fiber-reinforced metal component, a semifinished metal product or a metal coating, has no holes, cavities or pores which are free of matrix metal, since the metal wire meshes completely surround each fiber. By the use of fibers in the blank which are stable at high temperatures, thermal pressing processes may advantageously be used during further processing of the blank.
A preferred material for the fibers is silicon carbide, which not only allows maximum temperatures for further processing but also has an extremely high stability. In the preferred use of long silicon carbide fibers, blanks which can be wound up, and are extremely flexible and long may also be manufactured.
To improve the further processing of the blank, as well as its compressibility and the spacing of the fibers, the fibers are preferably long silicon carbide fibers with a metal coating made of the same metal or of the same metal alloy as the metal wire meshes. By means of such a metal coating, the packing density of the fibers can also be increased.
In a preferred construction of the blank, the metal wire meshes are linked by means of their tuck loops in a cross-sectionally triangular manner. This has the advantage that, in the center of gravity of the surface of each triangular link, a fiber may be arranged as the woof yarn. Because each triangular link is linked at its corners with an adjacent mesh or an adjacent tuck loop, the blanks can be advantageously deformed and can be adapted to a component or formed body, without any contact between or falling apart of, or shifting of the fibers or the coated fibers. Thus, the advantageous draping capacity of a knitted structure can be utilized to the manufacture fiber-reinforced metallic components.
A high packing density is achieved in a preferred embodiment of the blank in which the metal wire meshes supplement one another to form a uniformly thick blank of metal wire meshes that are triangularly linked to be standing on a vertex and a side. In this case, a lower metal wire forms the metal wire meshes which cross-sectionally stand on the side throughout a blank, and an upper metal wire forms the upper meshes which stand on the vertex, while the pertaining tuck loops alternately form a link between the upper and the lower metal wire. Therefore, very thin mats are advantageously manufactured as blanks, in which case the two fiber layers are arranged offset with respect to one another.
While the fibers are of an arbitrary length, the metal wire meshes are knitted of continuous metal wires in a preferred embodiment of the invention. This has the advantage that the blanks can be manufactured at reasonable cost on conventional industrial-scale knitting machines in a manner well known to those skilled in the art.
To further increase packing density, the fibers preferably form woof yarns placed in meshes, with the fibers which are surrounded by the lower metal wire meshes forming a lower layer, and the fibers which are embedded as the woof yarn into the upper metal wire meshes forming the upper layer. Higher packing density is achieved by means of the offset arrangement of the fibers between the upper and lower fiber layers.
In thick blanks, the fibers are staggered in more than two layers in the knitted material of meshes, with or without a tuck loop. In this manner, advantageously arbitrary thicknesses of the blanks can be achieved.
A preferred metal for the metal wire meshes is titanium. Titanium components are particularly light weight, and are commonly used in the construction of engines, which are subjected to high temperatures, aggressive gas flows and high tensile stress. Fiber reinforcement, for example, in the tensile direction, improves these characteristics and permits a high stressing of the components and a higher reliability of the power units.
In the manufacture of a blank, the fibers are preferably guided as woof yarns in a knitting machine in parallel to the longitudinal axis of the blank, and metal wire meshes are knitted around them in a conventional manner. For this purpose, the fibers are advantageously wound in the form of long fibers off supply spools and are guided to the knitting machine while, transversely to the woof direction, a thin metal wire is knitted around them in a close-meshed manner.
A preferred use of the blank according to the invention is for the manufacture of fiber-reinforced titanium plates, titanium rings or half-finished titanium products. A blank made of fibers and titanium wire meshes placed individually or in several layers or wound into a ring, is compressed by means of high-temperature isostatic pressing to form fiber-reinforced titanium plates, titanium rings or semi-finished titanium products. The plates, rings or semi-finished products are distinguished by the regular distribution of the fibers in a metal matrix made of titanium.
Another preferred use of the blank is for the manufacture of a self-supporting component made of fiber-reinforced titanium. For this purpose, one or more layers of a blank made of fibers and titanium wire meshes are pulled onto a core and compressed by means of high-temperature isostatic pressing; then the core is removed. In the case of an open structure this may be achieved simply by withdrawing the core as a single unit. For partially closed components, the core is either made to be divisible or is removed, for example, by burning-out or etching-out. The use of the blank according to the invention in this process has the advantage that the direction of the fibers in any position can be adapted to the constructional requirements, and even when fiber layers cross one another, contact between the fibers or a sliding of the fibers on one another is prevented. Furthermore, engine blade devices in the blade area may be manufactured from this material, advantageously permitting a high tensile load in the direction of the blade axis.
In addition, the blank may advantageously be used to manufacture self-supporting ring-shaped components, such as shrouds for turbine blade wheels with sealing tips and disk-shaped flanges. For such applications, a ring-shaped component is first wound from a blank by means of meshes consisting of titanium or of a blade material, and subsequently the sealing tips and disk-shaped flanges with titanium-coated or blade-material-coated long SiC fibers are wound onto or on the ring-shaped blank on negative forms. The long fibers and the blank are isostatically compressed at a high temperature to form a component. Because of the increased tensile loading capacity of these shrouds, it is possible for the first time to equip moving blade wheels with a co-rotating dynamically and thermally stable and closed shroud, and to aerodynamically seal the wheels by way of sealing tips arranged radially on the outside so that a further increase of the efficiency of gas turbine engines is achieved.
The blank according to the invention can also be used in coating processes. In this case, the blank is placed, wound, glued, soldered, diffusion-welded or spot-welded in one or several layers onto the component, and by means of a high-temperature isostatic pressing on the component is compressed to form a fiber-reinforced coating.
This coating process has the advantage that highly stressed components, such as engine shafts made of light metals, can be reinforced by means of a stability-increasing coating. Furthermore, in the blade area, the engine blade devices may be constructed to be thinner and therefore lighter.